Tag: 55 Cancri e

Since the first planet was discovered orbiting another Sun-like star in 1995, nearly 800 more have been discovered. Only a handful have been directly detected: most are discovered by their influence on their star, either by tugging it or blocking its light as the planet orbits (at the bottom of this post is a gallery of images of exoplanets detected in these ways). But some have been directly seen: either glowing by their own light, reflecting that of their star, or — ironically — seen when they’re not seen.

Say what? OK, this takes a sec to explain, but it’s cool.

The star 55 Cancri hosts at least 5 planets. Located 40 light years away, it’s one of the closer planetary systems, and has been intensely studied. One of the planets, 55 Cancri e, is bizarre: it’s twice the diameter of the Earth and has 8 times our mass. It’s thought to have a dense core surrounded by water… but Earth-like it ain’t. It orbits its star in a very tight orbit, circling it once every 18 hours. It’s so close to the star that the surface temperature is probably around 1700°C — or 3100°F! That’s hot enough to melt lead.

So yikes. If it does have water, it’s in the form of a weird super-heated steam only held to the planet due to its strong gravity. Even then, the atmosphere may be boiling away like a gigantic comet. So again, this isn’t like Earth at all. Even Venus isn’t this unpleasant, and on Venus it rains sulfuric acid.

Anyway, an object at that temperature will glow in the infrared, quite strongly. If it were sitting all by itself in space, it would be easy to see. However, it’s sitting next to a star which is millions of times brighter, making it a significantly more difficult target.

The problem with that is that the planet’s not terribly earth-like, and it may not be habitable*.

So what’s the deal? I read the journal article (PDF), and this really is a good story, just not the one I’m seeing the chatter about.

55 Cancri is a nearby binary star at a distance of about 40 light years. One star is a dinky red dwarf, and the other is a fairly Sun-like star, though somewhat smaller and cooler. It’s also much older, roughly 10 billion years old, more than twice the age of the Sun. It’s actually at the point where it’s starting to evolve into a red giant, and is called a sub-giant.

Back in 2007 it was announced that at least five planets orbit the bigger of the two stars (called 55 Cancri A; confusingly the red dwarf is 55 Cancri B (note the capital letter), while the planets are called b-f (lower case)). They range in mass from 0.026 to 3.84 times that of Jupiter (8.3 to 1200 times the mass of the Earth). 55 Cancri e is the lowest mass of these, but is extremely dense and hot, so not at all earth-like.

55 Cancri f is the interesting planet, though. The astronomers in question observed the star using an interferometer, allowing extremely precise measurements of the star’s size, which in turn yielded very accurate numbers for its temperature and mass. All these together can be used to figure out its "habitable zone", the region around it where an orbiting planet would have liquid water on its surface.

Now right away, I’ll say that finding the HZ (as we in the know call it) is not really straightforward. For example, a planet that has a thick atmosphere can be farther from its star and still have water due to the greenhouse effect; in fact, without air the average surface temperature of the Earth would be below freezing! And the greenhouse effect depends on what’s in the atmosphere, its density, and so on. So I am wary of any declarations of planets being habitable based on this alone.

Sometimes it pays to look over some older data and re-examine it. An exoplanet called 55 Cancri e was thought to have an orbit that was just 2.8 days long when it was discovered. However, two researchers looked over the data and realized they got a better fit if the orbit were actually only 0.73654 days — just under 18 hours! This meant it orbited its star far closer than previously thought as well.

And while that may be somewhat interesting, it’s the implications for the planet itself that make this orbital revision so cool. Or actually, hot. And dense.

Right. As usual, there’s a story to tell here…

The planet was discovered using the Doppler method: as it orbits its star, the gravity of the planet tugs on the star, causing a very small shift in the spectrum of starlight. The problem is getting enough observations to nail down the planet’s period; you can’t observe when it’s up during the day, and that cuts into the ability to get a good sampling of measurements. The discovery data gave a good fit at 2.8 days, so that’s what astronomers assumed was the orbital period.

But there were gaps in the data, and that can mask the true orbital period. When the data were examined more carefully, the 18 hour period was seen. But was it real?